Relationship between formula design and performance of high temperature resistant silica gel strip

A, rubber formula design and vulcanized rubber physical properties

(1) tensile strength

Tensile strength represents the ultimate ability of vulcanized rubber to resist tensile damage. Although the deformation of most rubber products will not be several times larger than the original length under service conditions, the actual service life of many rubber products has a good correlation with tensile strength.

The results of the study on the breaking strength of polymers show that the main valency bond of macromolecules, intermolecular forces (secondary valency bond) and the flexibility and relaxation process of macromolecule chains are the internal factors that determine the tensile strength of polymers.

The methods of improving tensile strength are discussed from various mating systems.

1. Relationship between rubber structure and tensile strength

The relative molecular weight of raw rubber is (3.0~3.5) ×105, which is beneficial to ensure high tensile strength.

When there are polar substituents on the main chain, the intermolecular force will increase, and the tensile strength will also increase. For example, the tensile strength of nitrile rubber increases with the increase of acrylonitrile content.

With the increase of crystallinity, the molecular arrangement will be closer and more orderly, which will reduce the pores and microscopic defects, strengthen the intermolecular force, and make the movement of macromolecular chain segments more difficult, thus improving the tensile strength. When the rubber molecular chain is oriented, the tensile strength parallel to the molecular chain increases.

2. Relationship between curing system and tensile strength

In order to obtain high tensile strength, the crosslinking density must be appropriate, that is, the dosage of crosslinking agent should be appropriate.

The relationship between the type of crosslinked bond and the tensile strength of vulcanized rubber decreases in the following order: ionic bond > polysulfide bond > disulfide bond > single sulfur bond > carbon-carbon bond. The tensile strength decreases with the increase of the bond energy of the crosslinked bond, because the weak bond with the lower bond energy can release the stress in the stress state, reduce the stress concentration, and make the crosslinked chain bear the larger stress uniformly.

3. The relationship between reinforcement filling system and tensile strength

The optimal dosage of reinforcing agent is related to the nature of reinforcing agent, adhesive type and other components in the formula: for example, the smaller the particle size of carbon black is, the greater the surface activity is, and the dosage tends to decrease when reaching the maximum tensile strength; When the carbon black content of soft rubber is 40~60 parts, the tensile strength of vulcanized rubber is better.

4. Relationship between plasticizing system and tensile strength

In general, when the amount of softener exceeds 5 parts, the tensile strength of vulcanizates decreases. For non-polar unsaturated rubber (such as NR, IR, SBR, BR), aromatic oil has little effect on the tensile strength of its vulcanized rubber. Paraffin oil has a bad effect on it; Naphthenic oil is somewhere in between. Low unsaturation of non-polar rubber such as EPDM, IIR, it is best to use low unsaturation paraffin oil and naphtha oil. For polar unsaturated rubber (such as NBR, CR), it is best to use esters and aromatic oil softeners.

In order to improve the tensile strength of vulcanized rubber, the choice of ancient Malone resin, styrene-indene resin, polymer oligomer and high viscosity oil is more favorable.

5. Other methods for improving tensile strength of vulcanized rubber

(1) Rubber and some resin blends such as NR/PE blend, NBR/PVC blend, EPDM/PP blend can improve the tensile strength of the blend.

(2) chemical modification of rubber by modifying agent in the rubber molecules or between rubber and filler to generate chemical bonds and adsorption bonds, in order to improve the tensile strength of vulcanized rubber.

(3) Packing surface modification using surface activity, coupling agent to treat the packing surface, in order to improve the interface affinity between packing and rubber macromolecules, not only contributes to the dispersion of packing, but also can improve the mechanical properties of vulcanized rubber.

(2) fixed elongation stress and hardness

Both fixed elongation stress and hardness are important indexes to characterize the stiffness of vulcanized rubber, both of which represent the force required by vulcanized rubber to produce a certain deformation. Constant tensile stress is related to larger tensile deformation, while hardness is related to smaller compressive deformation.

1. Relationship between molecular structure of rubber and fixed elongation stress

The higher the molecular weight of rubber, the less free end, the more effective chain number, the greater the stress of fixed extension.

All the structural factors that can increase the intermolecular force of rubber can improve the ability of vulcanized rubber network to resist deformation and improve the fixed extension stress. For example, structural factors such as polar atoms or polar groups on the main chain of rubber macromolecules and crystalline rubber increase the intermolecular force, so its fixed extension stress is high.

2. Relationship between curing system and constant extension stress

The crosslinking density has a significant effect on the fixed elongation stress. With the increase of crosslinking density, the fixed elongation stress and hardness increase almost linearly.

3. The relationship between filling system and fixed extension stress

The type and amount of filling are the main factors affecting the stress and hardness of vulcanizate.

The fixed elongation stress and hardness increase with the decrease of filler particle size, the increase of structure and surface activity, and the increase of filler dosage.

4. Other methods for improving the tensile stress and hardness of vulcanized rubber

(1) The use of phenolic resin/hardener, and rubber can generate three-dimensional space network structure, so that the schall A hardness of vulcanized rubber up to 95. For example, with alkyl resorcinol epoxy resin 15 parts/accelerator H1.5 parts, can be made of high hardness of the bead strip. (2) By adding liquid diene rubber and a large amount of sulfur into EPDM, high hardness vulcanizates with excellent curing properties and processing properties can be produced.

(3) Adding homogeneous polyester in NBR, NBR/PVC blend, NBR/ ternary nylon blend and other methods can make the schall A hardness of vulcanized rubber up to 90.

(3) Tearing strength

Tear is due to the vulcanized rubber crack or crack stress when the rapid expansion, cracking caused by the destruction phenomenon. Tear strength is the load per unit thickness of the sample when it is torn.

There is no direct relationship between tear strength and tensile strength, that is to say, vulcanizates with high tensile strength may not have high tear strength.

1. Relationship between rubber molecular structure and tear strength

With the increase of molecular weight, the intermolecular force increases and the tear strength increases. However, when the molecular weight increases to a certain extent, the tearing strength tends to be in equilibrium. The tearing strength of crystalline rubber is higher than that of amorphous rubber at room temperature.

The tearing strength of NR and CR is high at room temperature, which is due to the induced crystallization produced when the crystalline rubber is torn, which greatly improves the strain capacity. However, the tearing strength decreases obviously at high temperature except NR. The tearing strength of the vulcanized rubber was obviously improved after filled with carbon black.

2. Relationship between vulcanization system and tear strength

The tearing strength increases with the crosslinking density, but when it reaches the maximum, the crosslinking density increases and the tearing strength drops sharply.

3. Relationship between filling system and tear strength

The tearing strength increases with the decrease of carbon black particle size. In the case of the same particle size, the carbon black with low structure is beneficial to the tearing strength.

High tear strength can be obtained by using isotropic packing such as carbon black, white carbon black, White Yanhua, Lithopone and zinc oxide, etc. However, anisotropic fillers such as clay and magnesium carbonate cannot obtain high tear strength.

Some modified inorganic fillers, such as calcium carbonate and aluminum hydroxide modified with carboxylated polybutadiene (CPB), can improve the tear strength of SBR vulcanized rubber.

4. Effect of plasticizing system on tear strength

5. Generally adding softener will reduce the tear strength of vulcanizate. In particular, paraffin oil is very unfavorable to the tear strength of SBR vulcanizate, while aromatic oil can make SBR vulcanizate have higher tear strength with the increase of aromatic oil dosage.

(4) wear resistance

The abrasion resistance of vulcanized rubber is characterized by its ability to resist surface wear and material loss under the action of friction. It is a mechanical property closely related to the service life of rubber products. It is not only related to the use conditions, the surface state of the friction pair and the structure of the products, but also related to the physical and chemical properties of vulcanized rubber, such as other mechanical properties and viscoelastic energy, etc. There are many influencing factors.

1. Effect of rubber species

In general diene rubber, the wear resistance decreases in the following order: BR > solubilized SBR > emulsion SBR > NR > IR. The good wear resistance of BR is mainly due to its low glass transition temperature (Tg) (-95~105℃), good molecular chain flexibility and high elasticity. The wear resistance of SBR increases with the increase of molecular weight.

The wear resistance of NBR vulcanized rubber increases with the increase of acrylonitrile content, XNBR has better wear resistance than NBR.

Polyurethane (PU) is one of the most abrasion resistant of all rubbers. It has excellent wear resistance at room temperature, but its wear resistance decreases dramatically at high temperatures.

2. Influence of vulcanization system

The optimum wear resistance of vulcanized rubber depends not only on the curing system but also on the amount and structure of carbon black. When the amount and structure of carbon black are increased, the stiffness provided by carbon black will increase. In order to maintain the best value of the stiffness of vulcanized rubber, the rigid part provided by the vulcanized system must be reduced, that is, the crosslinking density should be appropriately reduced, and otherwise the crosslinking density of vulcanized rubber should be improved.

3. Influence of filling system

Generally, the wear resistance of vulcanized rubber increases with the decrease of carbon black particle size and the increase of surface activity and dispersion.

The wear resistance of the vulcanized rubber can be improved by the new process carbon black and the silica black treated with silane coupling agent.

4. Influence of plasticizing system

Generally speaking, the addition of softener in the adhesive will reduce the wear resistance. When aromatic oil is used in NR and SBR, the wear resistance loss is smaller than that of other oils.

5. The impact of the protection system

Under the condition of fatigue and wear, adding appropriate antiaging agent can effectively improve the wear resistance of vulcanized rubber. For example, 4010NA has a prominent effect, except 4010NA, 6PPD, DTPD, DPPD/H and so on all have a certain effect of preventing fatigue aging.

6. Other methods for improving the abrasion resistance of vulcanized rubber

(1) carbon black modifier add a small amount of carbon black modifier containing nitro compounds or other dispersants, can improve the dispersity of carbon black, improve the abrasion resistance of vulcanized rubber.

(2) vulcanized rubber surface treatment using halogen containing solution or gas, such as liquid antimony pentafluoride, gaseous antimony pentafluoride, NBR and other vulcanized rubber surface treatment, can reduce the friction coefficient of vulcanized rubber surface, improve wear resistance.

(3) the application of silane coupling agent modified filler such as the use of silica coupling agent A-189 treatment of silica black, filled in NBR rubber, the vulcanization rubber wear resistance is significantly improved, with silica coupling agent SI-69 treatment of silica black filled EPDM vulcanization rubber, its wear resistance can also be significantly improved.

(4) rubber and plastic blending rubber and plastic blending is one of the effective ways to improve the wear resistance of vulcanized rubber. For example, NBR/PVC, NBR/ ternary nylon can improve the abrasion resistance of vulcanized rubber.

(5) adding solid lubricants and anti-wear materials such as graphite, molybdenum disulfide, silicon nitride, carbon fiber, etc. in NBR rubber can reduce the friction coefficient of vulcanized rubber and improve the wear resistance.

(5) Elasticity

The high elasticity of rubber is caused by the change of conformational entropy of the coiled macromolecule.

1. The relationship between rubber molecular structure and elasticity

The larger the molecular weight is, the less the number of free ends that do not contribute to the elasticity is. The "quasi crosslinking" effect caused by entanglement within the molecular chain is increased, so the higher molecular weight is conducive to the improvement of elasticity.

A polymer composed of flexible molecular chains that are difficult to crystallize at room temperature. The more flexible the molecular chain is, the more elastic it is.

2. Relationship between curing system and elasticity

With the increase of crosslinking density, the elasticity of vulcanized rubber increases and reaches a maximum value, and then the crosslinking density continues to increase, while the elasticity decreases. Because moderate crosslinking can reduce the irreversible deformation caused by molecular chain slip, it is beneficial to increase the elasticity. Excessive crosslinking will result in blocked activity of molecular chain and decrease elasticity.

3. Relationship between filling system and elasticity

The elasticity of vulcanized rubber is completely caused by the conformational change of rubber macromolecules, so improving the rubber content is the most direct and effective way to improve the elasticity. Therefore, in order to obtain high elasticity, the dosage of filler should be reduced as far as possible, and the content of raw rubber should be increased. But in order to reduce the cost, the appropriate filler should be selected.

4. Relationship between plasticizing system and elasticity of vulcanized rubber

The effect of softeners on elasticity is related to their compatibility with rubber. The worse the compatibility of softener and rubber, the worse the elasticity of vulcanized rubber.

(6) Fatigue and fatigue damage

The relationship between strain resistance and adhesive species

According to the fatigue failure test of NR and SBR vulcanizates, when the strain is 120%, the relative advantages of NR and SBR are transformed. When the strain is less than 120%, the fatigue life times of SBR are higher than that of NR. When it is below 120%, it is below NR. NR has the opposite fatigue damage to SBR.

First, the relationship between rubber formula design and performance

(1) heat resistance

The so-called heat resistance refers to the ability of vulcanized rubber to maintain its original physical properties under the action of high temperature and long time thermal aging.

1. Rubber selection

A large number of studies have shown that the structural characteristics of heat-resistant polymers are: highly ordered molecular chains; Rigid; Having a highly rigid structure; High intermolecular force; Having a high melting or softening point. For example, polytetrafluoroethylene (PTFE), the use temperature of 315℃, fully in line with the above structural characteristics.

At present, EPDM, IIR, CSM, ACM, HNBR, FKM and Q are commonly used as heat resistant rubber.

2. Selection of vulcanization system

Different curing systems form different crosslinked bonds. The bond energy and oxygen absorption rate of each crosslinked bond are different. The higher the bond energy is, the better the thermal stability of the vulcanizate is. The slower the oxygen absorption rate, the better the oxidation resistance of vulcanized rubber.

Among the commonly used vulcanization systems, peroxide vulcanization system has the best heat resistance.

At present, the heat resistance of EPDM is almost peroxides vulcanization system. When peroxide is used alone as vulcanizing agent, there are some problems such as low crosslinking density and low thermal tearing strength. It is best to cross with some co-crosslinker.

3. Selection of protection system

Rubber products in high temperature use conditions, the antiaging agent may be due to volatilization, migration and other reasons for rapid loss, resulting in product performance deterioration. Therefore, in the heat resistant rubber formula, should use small volatile antioxidant or molecular weight antioxidant, it is best to use polymerization type or reaction type antioxidant.

4. Influence of filling system

The heat resistance of inorganic filler is better than that of carbon black, and the heat resistance of inorganic filler is better than that of white carbon black, zinc oxide, beauty oxide, aluminum oxide and silicate.

5. The effects of softeners

General softener molecular weight is low, volatile or migration at high temperature, resulting in increased hardness of vulcanized rubber, reduced elongation. So heat-resistant rubber formula should be selected under high temperature thermal stability, not easy to volatile varieties.

(two) cold resistance

The cold resistance of rubber can be defined as the ability to maintain its elasticity and function properly at specified low temperatures.

The cold resistance of vulcanizates mainly depends on two basic properties of polymers, namely glass transition temperature (Tg) T and crystallization.

The cold resistance of amorphous rubber can be characterized by Tg and Tb(brittleness temperature).

For crystalline rubber, Tg and Tb can not be used to characterize its cold resistance, which can be higher than Tg70~80℃.

1. Influence of rubber molecular structure on cold resistance

① Rubber containing double bonds and ether bonds in the main chain, such as BR, NR, CO, Q, has good cold resistance; ② The main chain does not contain double bonds, and the side chain contains polar groups